FIG. 1 of the accompanying drawings shows a generic liquid crystal display system which is an example of a flat panel display. Such a display is made up of an active area including an active matrix 14 which displays the image, a backlight system including a driver 12 to illuminate the display and a number of driver integrated circuits (ICs) 10 to control the addressing of pixels. The system is supplied with the display data 2, a number of control and timing signals 6 and power 4. These signals are typically supplied via a flexible printed cable (FPC). Attaching this cable adds a significant cost in the manufacture of the display. Display systems may also have integrated audio 250 and sensor 270 systems in addition to the active display area as illustrated in FIG. 2 of the accompanying drawings. Each one of these systems requires wireline connections to provide the system data (audio data 252, sensor data 266 and display data 2) and the relevant timing and control data (audio timing and control 254, sensor timing and control 268, display timing and control 6). The display system may have a single external power source 256 which may then connect to a series of voltage regulators 271 to meet the different voltage requirements (4, 258, 262, and 277). The increase in the complexity of the display system inevitably leads to a corresponding increase in the number of external connections which results in a bigger FPC connector. However, products incorporating these display systems are becoming physically smaller in size and there is therefore pressure to find alternative methods of transmitting the signals to the display system.
A wireless interface is a very attractive proposition. FIG. 3 of the accompanying drawings shows a generic wireless system comprising a data source 20 to generate the data, a transmitter system 22 which carries out the required formatting and signal processing, and a transmit antenna 24. These items generally form the transmitter side. The transmit antenna launches the signal as an electromagnetic wave or optical signal (depending on the implementation) through the wireless channel 26. On the receiver side, the receive antenna 28 couples the signal to the receiver system 30 which processes the data and passes it on to the data sink 32.
FIG. 4 of the accompanying drawings illustrates a generic transmitter system. The first block is a data source 160 to provide the actual content to be transmitted. Assuming the data is digital, it comprises a stream of ones and zeros where a high voltage level represents a one and a low voltage represents a zero. This coding scheme is called non-return to zero (NRZ).
This data may then be further processed and formatted 162 to match it optimally to the wireless channel. There are a number of coding schemes that can be employed at this stage. One of the most popular schemes is Manchester Encoding which is illustrated by the timing waveforms in FIG. 5a of the accompanying drawings. Manchester code is a self-clocking code with a minimum of one and a maximum of two level transitions per bit. In the Manchester data 304, a ‘Zero’ is encoded as a High-to-Low transition and a ‘One’ is encoded as a Low-to-High transition. Between two identical bits of data there is an extra level transition. It is normally implemented using the exclusive OR gate (XOR) function 306 between the data clock signal 300 and the NRZ data signal 302 as illustrated in FIG. 5b of the accompanying drawings. The XOR function of two variables is 1 if either of them but not both are 1.
After the data has been correctly formatted and encoded (FIG. 4), the next stage in the process is called modulation. A modulator 165 uses a data signal 167 to alter one of the properties of the high frequency carrier signal 163 generated by an oscillator 161. FIGS. 6a to 6e of the accompanying drawings illustrate typical modulation signals. FIG. 6a illustrates the high frequency carrier signal 163 and FIG. 6b illustrates the data signal 167. The parameters that are normally altered are one of the following: frequency, resulting in frequency shift keying (FSK) 170 (FIG. 6c); amplitude, resulting in amplitude shift keying (ASK) 174 (FIG. 6e); phase, resulting in phase shift keying (PSK) 172 (FIG. 6d). All other modulation schemes are derived from these three basic schemes. There are several methods available to implement these schemes. FIGS. 7a to 7c of the accompanying drawings summarise possible implementations of simple ASK, PSK and FSK. An amplitude modulated signal 174 is obtained by mixing a carrier signal 163 and a data signal 167 (FIG. 7b). A frequency modulated signal is obtained by switching between two carrier signals (f1 163 and f2 171) depending on the data signal 167 (FIG. 7a). In phase shift keying 172, the phase of the carrier signal 163 is varied between two values depending on the data signal 167 (FIG. 7c).
The modulated signal occupies a certain amount of frequency spectrum. This is a function of the modulation type being employed. The frequency components present in the modulated signal can be identified by computing the Fourier transform of the signal. This plot of the power available in the frequency components is known as the power spectral density (PSD) of the signal. It then leads to the definition of the signal bandwidth (BW). Although there are many acceptable definitions of bandwidth, it is generally referred to as the amount of spectrum occupied by all spectral components which have a power level of at least half the maximum level. The bandwidth of the modulated signal is directly related to the speed of the data signal. A high data rate requires more bandwidth.
The next stage in the transmitter (FIG. 4) involves amplifying the modulated signal to make it strong enough to be launched across the wireless channel. This is achieved using a power amplifier 164 which takes the low power modulated signal and produces a high power signal. Several methods for achieving this exist and examples are disclosed in Behzad Razavi, “Design of Analog CMOS Integrated Circuits”, McGraw-Hill 2001.
After the signal has been amplified, it is then launched into the wireless channel using an antenna 166. An antenna transforms the signal from a coupled electromagnetic wave into a radiated one and is designed to optimise power transfer in the frequency range of interest. The radiation will often be required to have a specified directionality i.e. maximise radiation of the signal in a specific direction.
Care must be taken in the design of the power amplifier 164 and the transmit antenna 166 to ensure that they have sufficient bandwidth to handle the modulated signal. If the bandwidth available in these systems is less than the signal bandwidth, then the signal will lose some information and can lead to erroneous decoding of the transmitted data.
The radiated signal travels across the wireless channel 26 (FIG. 3) after which it couples to the receive antenna 28. The receive antenna is designed to capture as large a signal as possible from the transmitted signal. Like the transmit antenna, it is designed to be efficient over the specified frequency range and direction.
The signal from the receive antenna then passes to the receiver system 30 whose main function is to extract the original data signal from the received signal. The exact implementation of the receiver system depends on the modulation scheme that was employed at the transmitter. FIG. 8d of the accompanying drawings shows a wireless receiver system using an amplitude shift keying modulated signal as an example. A generic wireless receiver system comprises a receive antenna 28, a demodulator 311 which extracts the transmitted data signal and a pulse shaping system 314 to convert the analogue data signal into a digital signal. The output 315 of the receiver system is the same as the data signal 167 that was used to modulate the carrier in the transmitter system (FIG. 4). In an amplitude modulated signal 174 (FIG. 8a), the data signal is embedded in the envelope (outline) 313 of the modulated carrier. Therefore the main role of the receiver, in this case, is to extract the envelope of the modulated carrier. This is carried out by first of all rectifying the amplitude modulated carrier which results in the output 310 (FIG. 8b) only having the positive going part of the original amplitude modulated signal. The rectified signal is then passed through a low pass filter 312 to remove the high frequency carrier signal. The last stage then involves pulse shaping 314 the low pass filtered signal into clean digital signal 315 (FIG. 8c) with defined high and low voltage levels.
There are many ways of implementing the pulse shaper. An example is illustrated in FIG. 9 of the accompanying drawings. A self-biased comparator circuit may be used to generate a digital output signal 402 from an analogue input signal 400. The high and low output voltage levels 404 and 406 are set during the design of the circuit. An arrangement of this type is disclosed in R. Jacob Baker, Harry W. Li, David E. Boyce, “CMOS, Circuit design, Layout and Simulation” IEEE Press, 1998.
The output signal 315 (FIG. 8c) at this point represents the data signal that was used to modulate the carrier at the transmitter side.
All systems employing wireless communication to transfer data are based on the description provided above. The systems will generally differ in the frequency of operation, data rate and modulation schemes employed. Wireless systems also increasingly have extra features such as error correction and perhaps encryption to maintain the integrity of the transmitted signal.
In certain applications, it may be desirable to transmit power wirelessly as this can reduce the number of wireline connections required by a system. This is normally achieved by transmitting power in the magnetic field of one coil and coupling it into a second coil. A generic scheme for transmitting power is illustrated in FIG. 10 of the accompanying drawings. Power transmission is achieved by coupling the magnetic field 34 of coil A 36 to that of coil B 38. Coil A is supplied with a power signal 35 in the form of an alternating current. The output signal 37 in coil B will also have an alternating current. The next step is to rectify this signal as illustrated in FIG. 11 of the accompanying drawings. A full wave rectifier 354 can be used to obtain an output signal 352 of single polarity (0 to +Vp) from a dual polarity (−Vp to +Vp) input power signal 350. At this point the signal is not a proper direct current power signal as it contains ripples. These can be removed by using some form of smoothing circuitry. This could be a large capacitor connected across the output signal. Once the relatively ripple free power signal has been obtained, it can then be regulated and stepped down to the required voltage levels.
To improve the efficiency of the power transfer using magnetic coupling, resonance circuits can used. The efficient transfer of power wirelessly can be achieved using series resonance at the transmitter and parallel resonance at the receiver. The series resonance at the transmitter maximises the current in the circuit, which in turn maximises the coupling magnetic field. The parallel resonance at the receiver maximises the voltage. Two types of such circuits are shown in FIGS. 12a and 12b of the accompanying drawings. The resonance of a series circuit (FIG. 12a) with resistor 40 of resistance R, inductor 42 of inductance L, and capacitor 44 of capacitance C, occurs when the inductive and capacitive reactances are equal in magnitude but cancel each other because they are 180 degrees apart in phase. Series resonance results in maximum current flowing through the circuit whose value depends on the resistance R. The sharpness of the resonance depends on the value of R and characterizes the “Q” of the circuit. The quality factor, “Q”, is a property of the inductor and is given by:
  Q  =            ω      ⁢                          ⁢      L              r      s      
where ω is the angular frequency (2πf) and rs is the equivalent series resistance of the metal windings of the inductor. The quality factor is a measure of the inductor's ability to store energy in its magnetic field such that a high Q results in a large current or voltage at resonance and efficient power transfer. FIG. 13 of the accompanying drawings shows examples of a response with high Q 390 and low Q 392.
Parallel resonance in a circuit with capacitor 46 of capacitance C, inductor 48 of inductance L, and resistor 50 of resistance R occurs at the frequency when the reactance due to the inductor is equal and opposite to the reactance due to the capacitor. Parallel resonance results in a maximum voltage across the resistor. The value R of the resistor 50 dictates the value of this voltage. Power transfer at resonance is more efficient as the voltages or currents at resonance are maximized as illustrated in FIG. 14 of the accompanying drawings, which plots the variation of the coupling magnetic field 65 with frequency 64. At the resonance frequency 60, the coupling magnetic field 65 is maximum and therefore the power transfer will be maximum at this frequency.
Wireless transmission of data alone is well known. The same can be said of wireless power transfer alone. However, the difficulty arises when an application requires both power and data to be transmitted wirelessly across the same wireless channel. Transferring data requires the data signal to somehow “piggyback” or be carried on the power signal. An example of such a technique is using the carrier signal to transfer the power and then using Amplitude Shift Keying modulation to couple the data to this carrier as illustrated in FIG. 15 of the accompanying drawings. The amplitude of the power carrying signal 74 is modified by the data carrying signal 76. This results in a composite transmitted signal 78 which carries both the power and data. The receiver for this scenario is different from the generic receiver illustrated in FIG. 8 as it now has to extract both the power and data from the transmitted signal. FIG. 16 of the accompanying drawings shows an example of such a receiver system. The transmitted signal is coupled to the receiver system 222 via a receive antenna 130. This is then followed by two rectifiers (power rectifier 134 and data rectifier 140) to extract the power and data signals. The output of the power rectifier is an unregulated voltage which is regulated by voltage regulators 136 to provide the required voltage level, VR 138. In the data signal extraction path, the output 141 of the data rectifier 140 is connected to a demodulator 144 which produces an analogue version of the data signal. This is then passed through a pulse shaping stage 145 to produce a digital signal 146 which is the same as the transmitted data signal.
Mathematically, ASK is a multiplication process of the carrier signal with the data signal in the time domain. Sometimes it may be necessary for the depth of the envelope (also called modulation depth) to be variable. In this case, a dc component is added to the data signal before the multiplication process is carried out. If the power carrying signal 74 is represented by equation,xc=cos(2πfct),
the data carrying signal 76 by equationxdata=cos(2πfdatat),
then the resulting transmitted signal 78 is represented, mathematically, byxtrans=(A+xdata)xc=A cos(2πfct)+cos [2πt(fc−fdata)]+cos [2πt(fc+fdata)]
where the depth of modulation is represented by a dc component A, fc is the frequency of the power carrying signal and fdata is the frequency of the data carrying signal.
This implies that, in addition to the carrier (power carrying) signal component fc, the transmitted signal will have frequency components at (fc−fdata) and (fc+fdata). FIG. 17 of the accompanying drawings shows the components of the transmitted signal in the frequency domain. The frequency components (fc−fdata 68) and (fc+fdata 62) are termed the sidebands of the transmitted signal. If the data carrying signal 76 has bandwidth, BW, the total bandwidth required to transmit both the sidebands is 2×BW 72.
For successful recovery of the power and data at the receiver side, the carrier and the two sidebands should be linearly transmitted across the wireless channel. In other words, the received signal should be directly proportional to the transmitted signal. The system transfer function (shape of the transmitter resonance curve) should, therefore, be such that it passes the signal without any distortion. FIG. 18 of the accompanying drawings shows the power carrying signal 60, the two sidebands 68, 62 and the system transfer function curve 67. For successful recovery at the receiver, the two sidebands should lie within the envelope of the transfer function curve 67. If the sidebands lie outside the resonance curve 67, such as at 61 and 63, data recovery will be impossible. The implication is that it is more difficult to recover both power and data if the data signal has high bandwidth. The equation relating this bandwidth limitation to the quality factor (Q) of the resonance curve is given by:Q=fc/(2×BW)
Increasing Q improves the power transferring capability of the system but reduces the overall bandwidth of the data that can be transmitted. This makes the transmission of both power and high bandwidth data a very challenging problem.
A method of meeting this challenge was suggested in “A wideband Frequency-Shift Keying Wireless Link for Inductively Powered Biomedical Implants”, M. Ghovanloo and K. Najafi, IEEE Trans. On Circuits and Systems, vol. 51, No. 12, December 2004. The approach taken by these authors is to shape the transfer function curve so that it passes the required frequency components in the transmitted signal without reducing the Q of the system. The authors use both series and parallel resonance circuits (FIG. 19 of the accompanying drawings) to produce two peaks in the transfer function to allow the power and the data carrying signal to be transmitted (FIG. 20 of the accompanying drawings). The system uses a form of Frequency Shift Keying (FSK) by transmitting data bit ‘0’ at one frequency and data bit ‘1’ at another frequency. A major drawback of this method is that the frequency at which the data bits ‘1’ and ‘0’ are transmitted should be highly stable. This is because the high Q of each peak in the transfer function of FIG. 20 results in a locally narrow band system and any deviation of the signal components from these frequencies will completely corrupt the transmitted signal. In FIG. 20, fdata 69 and fc 60 should therefore lie exactly at the centre of the respective peaks of the transfer function. This is very difficult if the Q is made very high to allow sufficient power transfer.
Another drawback of the system is that only one element of the antenna transmits the signal. In FIG. 19, this corresponds to Lp 120. The other inductor Ls, 116 does not transmit at all and just functions as a transfer function shaper. As the amount of power transfer the system can handle is related to the total inductance of the system, this will mean that the amount of power is limited to the amount that the single inductor Lp 120 can transmit. The actual implementation of this system is shown in FIG. 21 of the accompanying drawings where Ls 116 only has the function of shaping the signal and Lp 120 is the only transmitting element. This arrangement would suffice for low power systems. However, for high power system requirements such as powering the backlight of an LC display, a single transmitting element would not supply sufficient power.
Another known arrangement is disclosed in U.S. Pat. No. 7,071,629 B2 (FIG. 22 of the accompanying drawings). This system claims to be able to transmit both data 85 and power 84 wirelessly to a display system 81. This is implemented using a wireless transmitter element 90 and a receiver element 83 which incorporates a data and power extractor. However, the system is only able to transmit sufficient power to supply the driver ICs 87 and 89. The system still requires power to be supplied externally for the high voltage requirements of the display system in the form of HV 82 and GND 80 and cannot therefore be described as being totally wireless.